Many of the flaviviruses, including West Nile virus (WNV), cause significant human morbidity and mortality throughout the world resulting in high costs due to lost productivity and for extended health care. The final outcome of a viral infection is the result of a complex interaction between multiple host and viral components. A natural genetic variation in mice provides a unique model system for studying susceptibility at the molecular level. A single gene in mice (Flv) alters both the level of flavivirus production and disease outcome. This gene was identified during the previous funding period as 2'-5'oligoadenylate synthetase (Oas) 1b. Oas genes function as part of the innate immune response, producing 2-5A which activates the latent endoribonuclease, RNase L. Both 2-5A nor RNase L are virus non-specific. However, the effect of the product of the resistant Flv allele (Oaslbr) is flavivirus-specific and we have shown that this protein is not a functional 2'-5'oligoadenylate synthetase. The mechanism by which Oaslbr confers the flavivirus resistance phenotype is not known. Based on novel preliminary data, we hypothesize that Oaslbr can modulate activation of a rapid cell signaling response to flavivirus infection, that the set of genes up-regulatedcreates a cell environment less supportive of viral RNA replication and that Oaslbr can interact with the viral genomic RNAto reduce viral RNA genome replication. The aims propose studies to dissect the molecular pathway(s) through which the murine Oaslbr protein confers its flavivirus-specific inhibitory effect. Oaslbr may accomplish these various effects either directly or indirectly. Under Aim 1, we will investigate the RV- specific early signaling and gene up-regulation responses to WNV by identifying the componentsand pathways activated, by analyzing the transcription factors/activators regulating the up-regulated genes and by investigating the involvement of Oaslbr in these responses. Under Aim 2, we will identify and characterize cell protein and viral RNA binding partners of Oaslbr and analyze the possible roles of these proteins in modulating the resistance phenotype.These studies are expected to result in the discovery of new functional pathways for Oas/OAS proteins as well as of novel innate immune antiviral pathways. This research is relevant to understanding individual variation in the response to flavivirus infections among humans.